1. Field of the Invention
This invention relates to communication systems and more particularly to orthogonal frequency division multiplexing (OFDM) communication systems. These systems can include wireless or wired systems.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a digital multi-carrier modulation technique that uses a large number of orthogonal subcarriers for combating the effects of intersymbol interference (ISI), and achieves high data rates. In wireless communication systems, ISI is mostly a consequence of multipath fading. Multipath fading results from signal propagation over different reflective paths between a transmitter and receiver. In a deteriorating form, it introduces frequency selectivity in the band of interest and hence inhibits the usage of higher data rates.
Different types of wireless local area networks (wireless LAN), known as Wi-Fi and standardized by various IEEE 802.11 committees, are examples of networks that can employ OFDM techniques in the physical layer of the network architecture.
A network compliant with the advanced IEEE 802.11n standard uses multiple-input-multiple-output (MIMO) techniques to further increase the achievable data rates and reliability. That network can operate in 2.4 and 5 GHz unlicensed bands. Network equipment that operates in 2.4 GHz band is particularly susceptible to various interference sources such as microwave ovens, cordless telephones, Bluetooth devices and other appliances using the same band.
In wireless LAN networks, information is usually transmitted in packets. At the physical layer, those packets are pre-pended with a preamble so that the receiver is able to detect transmission of valid OFDM signal. Furthermore, sharing of the medium is controlled by the network layer called medium access control (MAC). A popular MAC protocol is a carrier-sense-multiple-access with collision avoidance (CSMA/CA) used in IEEE 802.11 applications. MAC defines Inter Frame Spacing during which a receiver should not see any signal energy if there is no external interferences present. OFDM is also used in wired networks, such as in digital subscriber line (DSL) networks and power line networks, which can be subject to interference.
FIG. 1 illustrates a simple IEEE 802.11n LAN that has 3 client stations 102 and a wireless access point 104. The interference source in the illustrated example is a cordless phone 106.
If an interferer (interference source) transmits its signal in the wireless LAN band, it can significantly slow down the network information throughput. Hence, there is a need for methods that can diminish the interference problem.
The presence of an external interferer can be easily detected during time slots when there should not be any signal energy in the medium.
FIG. 2 is a block diagram of a typical transmitter chain used in a multiple-input and multiple-output (MIMO) wireless LAN network such as the one presented in FIG. 1.
Information bits coming from the data source 202 are passed through a scrambler (SCR) 204 and forward error correction (FEC) 206. Many OFDM wireless systems use convolutional coding for FEC 206 at the transmitter and Viterbi algorithm (VA) decoder at the receiver. Besides convolutional coding, other types of error correction such as turbo-codes and block codes can be used.
After the FEC encoding, the data stream is divided into Nss spatial streams (SS) by a spatial parser 208 if spatial multiplexing is used. Next are interleaving 210 and modulation 212, which are followed by a block 214 that performs any of the desired MIMO techniques like beam-forming (BF), space time block coding (STBC) or diversity combining (DIV). The output of this block are Ntx transmit streams, that are converted to time domain by inverse Fourier transform, such as by inverse fast Fourier transform (IFFT) 216. Then, a cyclic prefix (CP) 218 is added to the symbols and finally they are passed to the radio frequency (RF) blocks 220 and antennas.
While the transmitter of FIG. 2 has Nss=Ntx=2, in general, those two numbers can be different as long as Nss<=Ntx, depending on the MIMO technique that is to be applied. Furthermore, both Nss and Ntx can take any positive integer value. Ntx=1 reduces to a single stream case.
The diagram of FIG. 2 shows just the basic blocks of the typical transmitter. It is well known to those skilled in the art that transmitter operation also includes: generation of preamble, generation of pilot symbols used for channel estimation and other receiver functions, etc. However, those details are not essential for the description of the current invention.
The inverse operations are done in the receiver and are illustrated in FIG. 3. The signals received at the antennas are passed through RF blocks 302 and cyclic prefix removal 304. The RF blocks 302 can correspond to a receiver front-end. The RF blocks 302 may generate baseband signals or intermediate frequency (IF) signals as outputs, which are passed to the next stage for processing. They are converted from time to frequency domain by the corresponding Fourier transform blocks, such as fast Fourier transform (FFT) blocks 306 and passed to an equalizer block 308. Equalizer block 308 includes channel estimation, frequency offset compensation with timing phase tracking, equalizer adaptation and equalization. The outputs of the equalizer block 308 are the compensated received streams and estimates of the quality of those streams.
The receive equalizer can be a linear minimum-mean square error (L-MMSE) equalizer, while the signal quality estimation is based on signal-to-interference-plus-noise ratio (SINR) for each subcarrier. Noise on each subcarrier generally comes from co-channel interference, inter-carrier interference (ICI), inter symbol interference (ISI) and thermal noise and other sources of noise and interference, which is included in signal to interference plus noise ratio (SINR) computation.
Usually, SINR includes the calculation of the subcarrier signal magnitude which is obtained from the channel estimate during the preamble. A strong interference signal during the preamble can produce a high magnitude, while the signal quality is poor. In other words, the reliability of the output SINR is dependent on the reliability of the corresponding subcarrier channel estimate, so interference during the preamble channel estimation portion of the packet can impair subsequent receiver performance, even if the interferer is no longer present. Other types of signal quality estimation may be used alternatively to SINR.
The equalized signal is passed to the soft demodulator block 310. Soft demodulation is preferred because soft decision Viterbi algorithm decoding with 8-bit quantization produces 2 dB of coding gain in Gaussian channel when compared to hard decision Viterbi algorithm decoding (see, for example, Sklar, Bernard, ‘Digital Communications’, Second Edition, Prentice Hall, 2001, p. 398). Furthermore, in contrast to hard decision, soft decisions can be weighted (W) with the signal quality estimation passed from the equalizer, which is good for the system performance in fading channels. Depending on the channel characteristics, weighted soft decision Viterbi algorithm decoding can significantly improve system performance in comparison to hard decision Viterbi algorithm decoding (see, for example, Heiskala, Juha and John Terry, ‘OFDM Wireless LANs: A Theoretical and Practical Guide’, Sams Publishing, 2002, pp. 113-116).
The weighting operation is done in the weighting block W 312 of FIG. 3. Next, the weighted soft decision signal is passed to deinterleaver block 314, deparsing blocks 316, Viterbi algorithm (VA) decoder 318 and finally to a descrambler 320.
One approach to mitigating interference is to increase the redundancy of forward error correction (FEC) that is used in the system. The more redundancy added, the greater the tolerance to interference and other noise effects. The disadvantage of this approach is that the capacity used to incorporate more redundancy is taken from payload data capacity. Furthermore, if the wireless LAN is to comply with a particular standard, the designer may not have the liberty to modify the selected coding scheme.
Another approach in handling interference is to do dynamic signal-to-noise ratio (SNR) measurements in order to identify degraded segments of the channel. See, for example, U.S. Pat. No. 6,990,059 to Anikhindi, et al. The degraded segments are then no longer used for transmitting payload data. Although this approach reduces the number of retransmissions in the system, a drawback is that it reduces information throughput.
Yet another approach is to dynamically select between two sets of L-MMSE equalizer taps depending on whether the received signal is dominated by noise or interference. See, for example, U.S. Pat. No. 7,012,978 to Talwar. This solution improves performance of L-MMSE equalizer, but does not completely eliminate presence of a strong interference signal in the Viterbi algorithm decoder processing.
Other methods that rely on estimating interference signal and subtracting it from the main signal path are possible. See, for example U.S. Pat. No. 6,999,501 by Sawyer. However, these methods may require some advanced knowledge about the type or statistics of the interfering signal.
There is a possibility of detecting and avoiding the interference by retuning the transmitter. See for example, U.S. Pat. No. 6,304,594 to Salinger. However, this approach could introduce significant delays in some cases or may be prohibited.
Some methods use statistical characterization of interference as received via the multiple channel outputs. See, for example, U.S. Pat. No. 6,757,241 to Jones, et al. Other methods use refined spatial statistical characterization starting from the initial estimate based on training symbols received via each antenna. See, for example, U.S. Pat. No. 7,095,791 to Jones, IV, et al. This statistical characterization may be computationally demanding.
It would be desirable to have a simple low-cost solution to improving the performance of wireless communication systems in the presence of interference.